MOS combustible gas sensors operate by catalytic oxidation of combustible gases at the "gate". Substantial efforts have been expended in recent years towards the development of combustible gas sensors using semiconductor MOS technology.
Generally, the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by a SiO.sub.2 insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gases. As a result of catalytic redox reactions on the gate surface, certain atomic and molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/semiconductor interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent A.C. admittance characteristic of the device along the voltage axis.
In order to be sensitive to combustibles other then H.sub.2, the catalytic gate and, therefore, the device have to be operated at temperatures above 400.degree. C., requiring the use of a wide band gap semiconductor, such as SiC instead of Si. However, at such high temperatures the SiO.sub.2 layer becomes less insulating as the ionic charges within the layer become mobile. Under these conditions, the device acts as a true capacitor only when biased in depletion (e.g., for n-type SiC, the gate voltage is negative with regard to the back contact). In accumulation, it begins showing some D.C. conduction. However, as the conduction process in the SiO.sub.2 is different from that in the SiC, there will be a finite voltage (the barrier potential) at which there is onset of this forward conduction. This barrier potential will depend on the charges injected into the SiO.sub.2 insulator by the chemical processes at the gate. As changes in this barrier voltage are directly reflected as changes in the current/voltage D.C. characteristic in the forward direction, a sensor response can be obtained by measuring changes in this characteristic as a function of changes in the combustible concentration near the catalytic gate. However, the mobility of charges in SiO.sub.2 is still relatively low. Therefore, to obtain a reasonable forward current with reasonable applied voltage, the thickness of this SiO.sub.2 layer has to be very small, leading to breakdown instabilities.